LED driver for VLC

ABSTRACT

An LED driver for driving a plurality of LEDs or LED groups to transmit light in a VLC mode is described, wherein the LED driver comprises a control unit that is configured to:receive, at an input terminal, a set-point representing a desired illumination characteristic and data that is to be transmitted using VLC;determine a current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile comprises a modulation in order for a corresponding intensity modulation as perceived by a sensor to represent a variable length VLC code representing the data, the VLC code comprising code words, a code word comprising a plurality of symbols;generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code; andupon receipt of a further set-point representing a different illumination characteristic during the transmission of the VLC code, determine an adjusted current profile or profiles for one or more of the plurality of LEDs or LED groups based on the different illumination characteristic, whereby the current profile is only adjusted for some, not all, symbols of a next code word to be transmitted;generate one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles, andoutput, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles.

RELATED APPLICATIONS

This application is a U.S. National Phase Patent Application ofInternational Application No. PCT/NL2018/050,087, filed Feb. 8, 2018,which claims priority to Netherlands Application No. NL 2018342, filedFeb. 8, 2017, and Netherlands Application No. NL 2019630, filed Sep. 26,2017, the disclosures of which are entirely incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to the field of light applications, in particularto LED-based lighting applications that are applied for visual lightcommunication (VLC).

BACKGROUND OF THE INVENTION

The present invention relates to visible or visual light communication(VLC) as may be applied by LED based lighting applications. In general,communication by means of visual light may be realized in variousmanners, using different modulation schemes to code the information thatis to be communicated. LED based lighting applications in general allowfor a high quality illumination experience, e.g. having variable colorfunctionality and dimming functionality. In addition, there areadditional demands that have to be met, e.g. with respect to flicker.Such a combination of demands may not always be met. In particular, itmay be difficult to maintain a broadcasting using VLC while anillumination set-point is adjusted. Or, it may be difficult to maintaina broadcasting using VLC substantially flicker-free. It may further bedesired to increase the amount of information or data that can bebroadcasted using VLC per unit of time.

It is an object of the present invention to provide in control methodsfor LED based lighting applications which provide in an improvedrealization of the various demands.

SUMMARY OF THE INVENTION

It would be desirable to provide in an LED driver enabling to moreaccurately operate in a VLC mode during a transient such as anillumination characteristic transient. It would also be desirable toprovide in an LED driver enabling to maintain operating in a VLC modeduring various circumstances. It would also be desirable to increase theamount of information or data that can be broadcasted using VLC per unitof time.

To address these concerns, in a first aspect of the invention, an LEDdriver is provided, the LED driving being configured to drive aplurality of LEDs or LED groups to transmit light in a VLC mode, whereinthe LED driver comprises a control unit that is configured to:

receive, at an input terminal, a set-point representing a desiredillumination characteristic and data that is to be transmitted usingVLC;

determining a current profile for each LED or LED group of the pluralityof LEDs or LED groups based on the desired illumination characteristicrepresented by the set-point, whereby an envelope of a combined currentprofile of the current profiles is modulated in order for acorresponding intensity variation as perceived by a sensor to representa variable length VLC code representing the data, the VLC codecomprising one or more code words, a code word comprising a plurality ofsymbols, whereby each of the plurality of symbols is characterized by arespective predetermined period;

generate one or more control signals to drive the plurality of LEDs orLED groups in accordance with the current profiles in order for theplurality of LEDs or LED groups to transmit the VLC code.

This first aspect may also be embodied in a visual light communication(VLC) method for a lighting system comprising an LED driver and aplurality of LEDs or LED groups, the method comprising the steps of:

receiving a set-point representing a desired illumination characteristicand data that is to be transmitted using VLC; and

determining a current profile for each LED or LED group of the pluralityof LEDs or LED groups based on the desired illumination characteristicrepresented by the set-point, whereby an envelope of a combined currentprofile of the current profiles is modulated in order for acorresponding intensity variation as perceived by a sensor to representa variable length VLC code representing the data, the VLC codecomprising one or more code words, a code word comprising a plurality ofsymbols, whereby each of the plurality of symbols is characterized by arespective predetermined period;

generating one or more control signals to drive the plurality of LEDs orLED groups in accordance with the current profiles in order for theplurality of LEDs or LED groups to transmit the VLC code.

In accordance with the first aspect of the present invention, an LEDdriver is provided for driving a plurality of LEDs or LED groups. In anembodiment, the LEDs or LED groups may e.g. have different illuminationcharacteristics such as a different color.

As an example, the plurality of LEDs or LED groups may comprises one ormore warm white LEDs constituting a first group and one or more coldwhite LEDs constituting a second group.

In an embodiment, the LED driver comprises one or more power converterfor supplying power to the LEDs or LED groups. Such power converters maye.g. comprise switched mode power converters such as Buck or Boostconverters which may be configured to generate a controlled outputcurrent in accordance with a control signal as received from a controlunit of the LED driver. Such a control unit may e.g. comprise one ormore processors or microcontrollers or the like, an input terminal forreceiving input signals representing a desired illuminationcharacteristic to be generated by the plurality of LEDs or LED groupsand an output terminal for outputting control signals for a powerconverter of the LED driver.

In an embodiment, the control unit may further be configured to receivedata or information that is to be transmitted by means of VLC by theLEDs or LED groups. Such data or information may also be received viathe input terminal or may be received via a dedicated data terminal ofthe control unit of the LED driver.

In accordance with the present invention, the LED driver is configuredto drive the LEDs so as to generate a variable length VLC code, wherebycode words, representing the data or information that is to betransmitted, are transmitted by means of a modulation of the overallintensity of the LEDs or LED groups. More details on such a code areprovided below.

In order to generate such a VLC code, the control unit of the LED driveraccording to the first aspect of the present invention is configured todetermine, e.g. based on a set-point representing a desiredillumination, a required current profile for each of the plurality ofLEDs or LED groups to generate the desired illumination characteristic,whereby an envelope of the combined current profile, i.e. thecombination of the current profiles of the plurality of LEDs or LEDgroups, comprises a modulation in order for a corresponding intensitymodulation as perceived by a sensor to represent a variable length VLCcode representing the data. The control unit may thus determine currentprofiles for the LEDs or LED groups, such that, when these currentprofiles are applied, an intensity modulation is obtained which can beperceived by a sensor. Further, in an embodiment, the current profilesare determined in such manner, that, on average, the illumination asgenerated by the LEDs or LED groups corresponds to the desiredillumination characteristic.

Within the meaning of the present invention, a desired illuminationcharacteristic may refer to a desired intensity, a desired color or thecombination of a desired color and intensity of the light source, i.e.the plurality of LEDs or LED groups.

In accordance with the present invention, the desired illumination maybe represented by a set-point, e.g. an intensity set-point, a colorset-point or a combination thereof.

In an embodiment of the present invention, the modulation as applied tothe combined current profile, i.e. the modulation corresponding to theenvelope of the combined current profile is such that both the desiredintensity and the desired color are maintained during the VLCtransmission. This can be realized in various manners:

A first manner to maintain both color and intensity is to select orconstruct the current profiles of the plurality of LEDs or LED groups insuch manner that the average intensity and average color as generatedduring the transmission of each symbol correspond to the desiredintensity and color.

A second manner to maintain both color and intensity is to select orconstruct the current profiles of the plurality of LEDs or LED groups insuch manner that the average intensity and average color as generatedduring the transmission of each code word correspond to the desiredintensity and color. This second manner enables a more flexible mannerto select or construct the current profiles, since there is no need tomatch the generated color and intensity for each symbol; rather, it issufficient to match the average intensity and color as generated duringthe transmission of a code word with the desired intensity and color.

Depending on the length of the code words as transmitted, it may even bepossible to only match the average intensity and color as generatedduring the transmission of multiple code words with the desiredintensity and color. When code words are sufficiently short, it may besufficient that the average intensity and color as generated during thetransmission of two or more code words corresponds to the desiredintensity and color, without an observer noticing the intensity or colorvariation during the transmission of the two or more code words.

In an embodiment of the present invention, a symbol as transmitted isnot only characterized by its duration or period, but also by a color orcolor characteristic applied during the transmission. By characterizinga symbol by both a particular duration or period and a color or colorcharacteristic, a much larger number of distinguishable symbols may begenerated, enabling the transmission of an increased amount of data orinformation per unit of time.

In such an embodiment, a receiver (e.g. a smartphone, tablet, laptop,computer or the like) may be configured to both assess an intensity, inparticular a variation of the intensity over time in accordance with theenvelope of the combined current profile as applied to the plurality ofLEDs or LED groups, and a color of the transmitted symbols and codewords.

Regarding the feature color or color characteristic as applied for thetransmission of a symbol or code word, various options may be applied.

In a first embodiment, the color associated with the transmission of asymbol or code word can be defined as the average color as observed by areceiver during said transmission. As an example, when a code wordconsisting of four symbols is transmitted using only red LEDs, this codeword may be given a particular meaning. When the same code word istransmitted using only Blue LEDs, the code word may have a differentmeaning. When the same code word is transmitted using Red LEDs for thefirst two symbols and using Blue LEDs for the last two symbols, it mayhave yet another meaning. Also, when a code word or symbol istransmitted with an average color which is a particular, predefinedmixture of the available Red and Blue light, the code word or symbol mayhave yet another meaning. As will be understood by the skilled person,by distinguishing between symbols or code words based on both durationand color, one obtains a larges set of possible symbols or code words,which enables to increase the information density that can betransmitted per unit of time.

In a second embodiment, a distinction between symbols having the samelength is made based on which LED or LED group is used to generate themodulation in the envelope of the combined current profile. Assuming alight source as controlled by an LED driver comprises a Blue LED and aRed LED, one may either, in order to generate a modulated currentprofile of which the symbol periods or durations can be detected by areceiver, modulate the light as generated by the Blue LED, or modulatethe light as generated by the Red LED, or modulate the light asgenerated by both the Red LED and the Blue LED. These three differentscenario's can be considered to represent three different symbols, whilethe period of duration of the symbols are the same. This secondembodiment thus also enables to improve the information density that canbe transmitted per unit of time.

As will be understood by the skilled person, in case the color or acolor characteristic is also used to characterize a particular symbol orcode word, they would in general be a mismatch between the desiredcolor, i.e. the color as requested by a user and e.g. represented by acolor set point, and the color as applied during the transmission of theVLC code. In order to overcome or mitigate this mismatch, variousoptions exist.

As a first option, the transmission of a code word using a particularcolor or color characteristic may be followed by a period during whichthere is no VLC transmission and during which, the color mismatch iscompensated or mitigated. Depending on the length or duration of a codeword or code words and depending on the required compensation, such acompensation may take place after every code word or after a pluralityof code words, e.g. after every two or three or more code words aretransmitted.

As a second option, rather than defining a different symbol for everydifferent color or for every color range available for a particularlight source, one may e.g. choose two or more different colors torepresent the same symbol. By doing so, one may, when such a symbolneeds to be transmitted, select the most appropriate color out of thedifferent colors that represent the symbol, the most appropriate colore.g. being closest to the desired color as defined by the color setpoint or the most appropriate color based on an average color asdetermined or generated so far, i.e. up to the instant when the symbolneeds to be transmitted. More detailed embodiments will be discussedfurther on.

With respect to modulations in intensity and color, as may be appliedduring the transmission of a VLC code, it can be noted that, in general,by ensuring that the intensity modulation occurs on a comparatively highfrequency, e.g. above 250 or 300 Hz, the modulation will not be noticedby an observer, rather, the observer will perceive the mixed, i.e. theaveraged intensity of the LEDs or LED groups. The same holds formodulations or variations in the applied color. It has however beenobserved that color variations may occur at lower frequencies beforethey are noticed by an observer.

In accordance with the present invention, the control unit of the LEDdriver according to the first aspect of the present invention is furtherconfigured to generate one or more control signals to drive theplurality of LEDs in accordance with the current profiles in order forthe plurality of LEDs or LED groups to transmit the VLC code. Thesecontrol signals may e.g. be provided to the one or more power convertersof the LED driver.

In accordance with an embodiment of the present invention, the controlunit of the LED driver is further configured to ensure that atransmission of a VLC code, e.g. representing data or information, isnot interrupted or disturbed, when an illumination characteristic of theLEDs or LED groups is altered or to be altered. In particular, in anembodiment of the present invention, the control unit of the LED driveraccording to the present invention is configured to, upon receipt of afurther set-point representing a different, desired illuminationcharacteristic, only gradually implement this new illuminationcharacteristic when a VLC code is transmitted.

In such embodiment, the control unit of the LED driver according to thepresent invention may be configured to perform the following steps:

upon receipt of a further set-point representing a differentillumination characteristic during the transmission of the VLC code,determine an adjusted current profile or profiles for one or more LEDsor LED groups of the plurality of LEDs or LED groups based on thedifferent illumination characteristic, whereby the current profile isonly adjusted for some, not all, symbols of a next code word to betransmitted;

generate one or more control signals to drive the plurality of LEDs orLED groups in accordance with the adjusted current profile or profiles,and

output, via an output terminal of the control unit, one or more controlsignals to drive the plurality of LEDs or LED groups in accordance withthe adjusted current profile or profiles.

In an embodiment, such a gradual implementation is realized bydetermining an adjusted current profile for one or more of the LEDs orLED groups, based on the different desired illumination characteristic,whereby only some of the symbols in a next code word that is to betransmitted take account of the new illumination characteristic. Assuch, a transition from a first illumination characteristic, e.g.represented by a first desired color and intensity of the LEDs or groupsof LEDs to a second illumination characteristic, e.g. represented by asecond desired color and intensity of the LEDs or LED groups isintroduced in multiple steps.

As an example, the control unit of the LED driver may e.g. be configuredto:

in a first step, determine current profiles for transmitting a code wordcomprising multiple symbols based in the first illuminationcharacteristic;

in a second step, determine one or more adjusted current profiles fortransmitting a second code word comprising multiple symbols based on thesecond illumination characteristic, whereby the some, but not allsymbols of the second code word take the second illuminationcharacteristic into account;

in third step, determine one or more further adjusted current profilesfor transmitting a third code word comprising multiple symbols based onthe second illumination characteristic, whereby the further adjustedcurrent profiles result, when applied to the LEDs or LED groups, in thesecond illumination characteristic.

In an alternative embodiment, rather than only applying an adjustedcurrent profile to only some of the symbols of a next code word that isto be transmitted, a desired transition between a first illuminationset-point or characteristic and a second illumination set-point orcharacteristic can be gradually introduced by applying one or moreintermediate current profiles to a sequence of code words.

In such arrangement, a control unit as applied in an LED driveraccording to the present invention may be configured to, after receivinga first illumination characteristic and determining the correspondingcurrent profiles to realize this illumination characteristic:

receive a further set-point representing a different second illuminationcharacteristic;

determine a further current profile for one or more of the plurality ofLEDs to generate the different second illumination characteristicrepresented by the further set-point,

determine one or more intermediate current profiles for one or more ofthe plurality of LEDs, and

determine output control signals for the LED driver to sequentiallydrive the plurality of LEDs or LED groups according to:

the current profiles when transmitting a first code word;

the one or more intermediate current profiles, when transmitting arespective one or more further code words, and

the further current profiles when transmitting a yet further code word.

In an embodiment, the intermediate current profiles may e.g. representillumination characteristics that are combinations of the first andsecond illumination characteristics.

As an example, the first illumination set-point may e.g. be to operate afirst LED group at 50% and the second LED group at 50%, whereas thesecond illumination set-point may e.g. be to operate a first LED groupat 70% and the second LED group at 30%.

In such example, the control unit may e.g. be configured to determineand sequentially apply:

current profiles corresponding to operate a first LED group at 50% andthe second LED group at 50% when transmitting a first code word;

current profiles corresponding to operate a first LED group at 60% andthe second LED group at 40% when transmitting a second code word;

current profiles corresponding to operate a first LED group at 70% andthe second LED group at 30% when transmitting a third code word.

In such embodiment, the control unit may thus implement one or moreintermediate illumination set-point, e.g. obtained by linearinterpolation between a first set-point and a second set-point, therebyobtaining a gradual transition between two set-points, during a VLC codetransmission.

It may further be pointed out that during a transition from a firstillumination set point and a second illumination set point the step ofdetermining one or more intermediate current profiles should takeaccount of whether a symbol or code word is only characterized by apredetermined duration or period, or is also characterized by its color.

In an embodiment of the present invention, the control unit as appliedin the LED driver according to the present invention is configured tosubstantially maintain a modulation depth during the operation in VLCmode when an illumination characteristic is adjusted.

In an embodiment, the frequency content of the current profiles asapplied during the transition from a first illumination set-point to asecond illumination set-point is kept above a predetermined value. Inparticular, in an embodiment, the adjusted current profiles orintermediate current profiles are constructed to have a lowest frequencythat is equal to or higher than a lowest frequency of the currentprofiles.

In accordance with a second aspect of the present invention, there isprovided a visual light communication method for a lighting systemcomprising an LED driver and a plurality of LEDs or LED groups, themethod comprising the steps of:

receiving a set-point representing a desired illumination characteristicand data that is to be transmitted using VLC; and

determining a current profile for each of the plurality of LEDs or LEDgroups based on the desired illumination characteristic represented bythe set-point, whereby an envelope of a combined current profile of thecurrent profiles is modulated in order for a corresponding intensityvariation as perceived by a sensor to represent a variable length VLCcode representing the data, the VLC code comprising one or more codewords, a code word comprising a plurality of symbols, whereby each ofthe plurality of symbols is characterized by a respective predeterminedperiod, the combined current profile having a high current period and alow current period during each predetermined period, and wherein thecombined current profile comprises, during the high current period orthe low current period of at least one of the predetermined periodscharacterizing a symbol, a current increment during a current incrementperiod that is less than the high current period or the low currentperiod.

Within the meaning of the present invention, a period or predeterminedperiod refers to a duration in the time domain, i.e. a length or portionof time.

Within the meaning of the present invention, a current increment refersto an incremental increase or decrease of a supplied current to an LEDor LED group during a specified period, whereby incremental increase ordecrease refers to a transition between two consecutive availablecurrent levels.

These and other aspects of the invention will be more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description and considered in connection with theaccompanying drawings in which like reference symbols designate likeparts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a current profile representing symbols to be used in avariable length VLC method.

FIG. 2 depicts a detailed current profile of a symbol to be used in avariable length VLC method.

FIGS. 3 and 4 a depict two possible current profiles for a first andsecond LED or LED group representing symbols to be used in a variablelength VLC method.

FIGS. 4b and 4c schematically depict CEI diagrams to illustrate visuallight communication methods according to the present invention.

FIGS. 5 and 6 depict current profiles for a sequence of two code wordsto be transmitted using VLC, the second code word having an amendedprofile.

FIGS. 7 and 8 depict alternative current profiles representing symbolsto be used in a variable length VLC method.

FIGS. 9 and 10 depict current profiles as can be applied by an LEDdriver according to the present invention during operation in VLC mode.

FIG. 11 schematically depicts an LED driver according an embodiment ofthe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 depicts a current profile which can be applied by an LED driverfor transmitting a code or code word using VLC (visible lightcommunication) in accordance with an aspect of the present invention. Ingeneral, a VLC scheme applies a plurality of different symbols, e.g.4-10, which can be combined into code words, each code word consistingof a combination of a predefined set of symbols. In such a VLC scheme,each symbol is represented by a particular current modulation, which mayresult in a modulated intensity when applied, which intensity variationmay be detected by a sensor. Known examples of such modulation areon-off keying (00K) and pulse position modulation (PPM). In the currentprofile as shown, a code word or code that is to be communicatedconsists of a combination of multiple symbols, whereby the symbols thatare used have a different length. In the example as shown, 4 symbolsS0-S3 are shown, each having a different duration or period T0-T3. Inthe example as shown, the current profile of each symbol has a firstperiod, also referred to as the high current period, during which thecurrent is comparatively high, followed by the second period, alsoreferred to as the low current period, during which the current iscomparatively low. As a result, a receiver having a light sensor may becapable, based on the intensity variations observed, to determine theinstants t0-t3 when the symbols start and end and thus, based on theperiods between these instants, identify the symbols and thus the codeword that is transmitted. In particular, a receiver capable ofdetermining the duration between two consecutive instants at which theintensity increases, would be capable to identify the symbols, based onthe determined duration. In order to ensure proper detection of theinstants that define the length of the symbols, a sufficient increase inintensity should be applied. In the present invention, the differencebetween the current at the end of a symbol and the current at thebeginning of a next symbol may be referred to as the modulation depthMD. It may be a measure indicative of the ease or accuracy with whichthe instants t0-t3 may be determined. In case the modulation depth MD istoo small, it may be difficult for a receiver or sensor to notice theintensity change.

In a practical implementation, periods T0-T3 of the symbols may e.g. bein a range between 200 μsec and 500 μsec, whereas the duration or periodof a code word is e.g. smaller than or equal to 3 ms or 3.33 ms.

With respect to the implementation of a VLC scheme, it may further beimportant to point out that, in general, the instants at which thecurrent level may be adjusted are fixed, e.g. fixed relative to a clockfrequency. As an example, the clock frequency can be such that a newset-point can only be applied upon expiring of a minimal predeterminedperiod. This minimal period may e.g. be referred to as the incrementalperiod, or clock period, W (see FIG. 2) and may be in a range between 20μsec and 30 μsec, e.g. 26 μsec. As such, the length or period of theparticular symbols S0-S3 as used in the coding will typically be amultiple of the incremental period W, e.g. S0=10×W, S1=14×W, S2=18×W,S3=22×W. In addition, the amplitude of the current I as shown in thecurrent profile will typically also be adjustable in a discrete manner,rather than a continuous manner. In particular, as shown in FIG. 2, thesmallest adjustment that can be applied to the current is the distancebetween two adjacent horizontal dotted lines, this smallest currentincrement being referred to, in the present invention, as theincremental current step ICS. Or, phrased differently, an incrementalincrease or decrease of a supplied current to an LED or LED group refersto a transition between two consecutive available current levels.

FIG. 2 schematically shows a current profile CP representing aparticular symbol, the symbol being represented or identifiable by meansof the period T of the symbol. In FIG. 2, the vertical dotted linesindicate instants at which the current I can be changed, the horizontaldotted lines indicate possible amplitudes of the current I.

In such an arrangement, an increase or decrease of the current I canonly occur along the dotted lines.

In case a current profile as shown in FIG. 1 or 2 is used to transmitinformation, it will be clear that this current profile will correspondto a certain illumination intensity. In order to avoid the modulation ofthe current to be perceived by an observer, the periods of the differentsymbols or the overall period of a code word should be selected to becomparatively low, such that the frequency content of the currentprofile is comparatively high. As an example, the periods of the symbolsused can be selected in such manner that the overall length or period ofa code word is less than or equal to a predetermined value, e.g. 3.3msec.

In addition to a desired intensity, an illumination set-point mayfurther include a desired color to be generated by the light source, thelight source e.g. comprising a plurality of LEDs emitting light of adifferent frequency or frequency spectrum. The color of the illuminationas perceived by an observer can be adjusted by adjusting the relativeintensity of the plurality of LEDs applied.

In case various LEDs of different color are applied in an LED fixture,VLC may still be implemented in a similar manner as described above. Inparticular, when differently colored LEDs, or at least some of the LEDs,are provided with a modulated current, the envelope of the overallcurrent profile can be used to represent the desired symbols forming acode word. In such case, the overall, or combined current profile willresult in an overall intensity variation which may be perceived by anoptical sensor. In order to realize this, a synchronization of themodulation applied to the different LEDs may be required.

FIG. 3 schematically shows current profiles applied to a first and asecond LED and the combined current profile. As shown, the currentprofile of the first LED, I1, is a continuous current with amplitude I1,the current profile of the second LED, I2 is a modulated current.Combined, the envelope of the current profile resembles the currentprofile of FIG. 1; i.e. the combined current profile may be consideredto represent the required current modulation for broadcasting foursymbols S0, S1, S2, S3. Note that, in line with FIG. 2, the currentamplitude for both the first LED and the second LED are modulated inaccordance with discrete values, as are the instants where the currentamplitude can be adjusted. In case the depicted current profiles areapplied in a first and a second LED, a sensor observing the lightgenerated by the combined current profile will be able to determine theinstants at which the amplitude of the current envelope changes (i.e.increase from a value I1 to a value I1+I2, and thus the symbols that arecommunicated via the modulation.

As will be understood by the skilled person, the color and intensityset-point as obtained by the current profiles shown in FIG. 3, may beobtained using alternative current modulations. FIG. 4a schematicallyshows an alternative manner to arrive at the same combined currentenvelope using alternative modulations of the currents I1 and I2, boththe alternative modulation of FIG. 4a and the modulation shown in FIG. 3resulting in the same intensity and color experience for an observer. Inthis respect, it is assumed that there is a linear relationship betweenthe current and the intensity.

With respect to realizing a particular illumination characteristic orset-point, i.e. characterized by both a color set-point and an intensityset-point, it can be pointed out that, depending on the period orduration of a symbol or code word, the realization of a desiredintensity and color may be realized per symbol, per code word or permultiple code word.

With respect to the sensing of the VLC, it can however be pointed outthat, although the modulation depth of the current modulations shown inFIGS. 3 and 4 a is the same, there may be a preferred manner to modulatethe currents I1 and I2, e.g. to take into account that a sensor ordetector that is used to detect the intensity changes may have adifferent sensitivity for different wavelengths.

In an embodiment of the present invention, the symbols as used to form acode word are only characterized by their duration or period, e.g.periods T0-T3 in FIG. 1. In such embodiment, the applied color duringthe transmission may be selected at will, e.g. corresponding to thedesired color as represented by a color set point of an illumination setpoint.

In another embodiment, a symbol as transmitted is not only characterizedby its duration or period, but also by a color or color characteristicapplied during the transmission. By characterizing a symbol by both aparticular duration or period and a color or color characteristic, amuch larger number of distinguishable symbols may be generated, enablingthe transmission of an increased amount of data or information per unitof time.

Various options exist to define a symbol by its color or a colorcharacteristic as applied during the transmission. This may e.g. beillustrated by reference to FIG. 4 b.

FIG. 4b schematically shows a CEI chromaticity diagram and positions ofthree LEDs 100.1, 100.2 and 100.3 of a light source in said diagram.

As will be understood by the skilled person, by proper control of theintensities generated by the three LEDs, one may generate substantiallyany color within the indicated triangle 110, e.g. color 105 asindicated.

As a first example how to characterize a symbol by a color (apart fromthe period or duration) is to use the available colors (100.1, 100.2 and100.3). by doing so, the number of distinct symbols that can betransmitted has tripled.

As a second example, the available color space, i.e. the area covered bythe triangle 110, can be subdivided into different areas. In FIG. 4b ,the different areas are defined/bound by the dotted lines 120. By doingso, one can e.g. characterize a symbol by both its period or durationand its position in the CEI diagram. In particular, each area as definedby the dotted lines 120, e.g. areas 120.1 or 120.2 can be used todistinguish between symbols. A symbol having a duration T0 which istransmitted with an average color within area 120.1 may thus bedistinguished from a symbol having a duration T0 with an average colorwithin the area 120.2. It may be pointed out that the degree with whichone may subdivide the available color space 110 to distinguish symbolsshould also take into account the possibility of a receiver todistinguish between different colors.

Using this approach, the available number of symbols may be increasedsignificantly, thus increasing the amount of information or data thatcan be transmitted per unit of time.

It may also be noted that the above may also be applied on a code wordlevel. In a similar manner as defining a symbol by the average colorapplied during the transmission of the symbol, one may also give adifferent meaning to code words, depending on the average color appliedduring their transmission.

When characterizing a symbol or code word to be transmitted by both itsduration and color, it will be apparent to the skilled person that thiscolor will often deviate from the desired color, e.g. color 105. Whenthis occurs, a color compensation scheme or method may be applied in thevisible communication method according to the present invention.

As an example, assume that a plurality of code words, each comprisingmultiple symbols, are to be transmitted using colors within the areas120.1 and 120.2 and, at the same time, the desired color set pointcorresponds to the color indicated by reference number 105.

In such example, as will be understood by the skilled person, thetransmission of the symbols and code words will be primarily obtained byusing the LEDs 100.1 and 100.2, whereas, in order to generate color 105,LED 100.3 would have to be applied the most.

In order to arrive, on average, at a desired color, a color compensationscheme may e.g. comprise interrupting the VLC process after each codeword or N code words and applying, during a predetermined period (whichmay e.g. depend on the duration of the VLC transmission and the colorapplied during said transmission) a different color. During such idletime, i.e. when no VLC code is transmitted, the currents as applied tothe LEDs or LED groups may e.g. be substantially constant.

Alternatively, one may continue to operating in a VLC transmission modeduring the predetermined period used for compensating the color, wherebythe transmission involves transmitting one or more code words that haveno informational meaning.

In the given example, the color compensation may e.g. involve togenerate, during a predetermined period, an illumination primarily usingLED 100.3 after the transmission of one or N code words primarily usingLEDs 100.1 and 100.2, in order to arrive at an average color 105.

As will be understood, the number of code words that can be transmittedbefore a compensation is required may depend on the difference betweenthe applied color during the VLC transmission (e.g. colors 100.1 and100.2) and the desired color 105. Based on this difference (which cane.g. be referred to as color-error), one may decide to compensate colorafter every code word or after every N code words.

An alternative or additional method to enable the generation, onaverage, of a desired color set point when applying a VLC methodaccording to the present invention is illustrated with reference to FIG.4 c.

FIG. 4c schematically shows a CEI chromaticity diagram including acontour 130 known as the black body line indicating white colors rangingfrom cold white (CW) to warm white (WW), which may e.g. be generated bya particular combination of LEDs.

In an example, it is assumed that a desired color set point willsubstantially correspond to a color at or near the contour 130.

In order to increase the number of different symbols that may betransmitted, a similar approach as discussed with reference to FIG. 4bcan be applied, whereby, in this case, the contour 130 is subdividedinto segments, each segment indicating a color range that can be used todistinguish between symbols. As an example, when the contour 130 coversa white color ranging from 2400 Kelvin (K) to 3400 K and is subdividedin 10 segments (indicated by references R1-R10), each spanning a rangeof 100 K & 4 different durations or periods (T0-T3) are used tocharacterized symbols, 4×10=40 distinct symbols could be generated. Incase a code word would consist of 4 symbols, one could thus generate 40⁴different code words.

In order to facilitate the compensation of a color-error, i.e. occurringwhen the symbols or code words that are to be transmitted have adifferent color than the desired color, one can attribute two or moredistinct colors or color ranges to the same symbol. For the givenexample where the contour 130 is subdivided into 10 segments or colorranges, one could e.g. define the color range CR for a particular symbolS as a combination of two segments.

A symbol S0 could then be characterized by its duration T0 and a colorin the range R1 or R10.

A symbol S1 could then be characterized by its duration T1 and a colorin the range R2 or R9.

A symbol S2 could then be characterized by its duration T2 and a colorin the range R3 or R8.

A symbol S3 could then be characterized by its duration T3 and a colorin the range R4 or R7.

A symbol S4 could then be characterized by a duration T4 and a color inthe range R5 or R6.

By doing so, one can transmit a symbol S0 by either applying a color inthe range R1 or in the range R10.

As will be appreciated by the skilled person, this increased flexibilityin selecting a color to transmit a particular symbol, may facilitate inmaintaining an average applied color close to a desired color.

Applying the above method may limit the number of symbols that can begenerated, compared to the situation where each range R1-R10 is used toindicate a different symbol. However, it may reduce the idle time, i.e.the time during which no VLC transmission can be performed because acolor compensation needs to be applied.

In the above example, the symbol S0 is characterized by having a colorin either range R1 or range R10, i.e. the two most extreme segments ofthe contour 130. By selecting segments to characterize the color of asymbol that are remote enables to select the most appropriate color totransmit a symbol, given a desired color set point. In the givenexample, the color difference between segments associated with the samesymbol (e.g. difference between R1 and R10 or the difference between R2and R9) varies per symbol. An alternative definition of the color rangefor the different symbols could arrange for a substantially constantdifference:

A symbol S0 could be characterized by its duration T0 and a color in therange R1 or R6.

A symbol S1 could be characterized by its duration T1 and a color in therange R2 or R7.

A symbol S2 could be characterized by its duration T2 and a color in therange R3 or R8.

A symbol S3 could be characterized by its duration T3 and a color in therange R4 or R9.

A symbol S4 could be characterized by a duration T4 and a color in therange R5 or R10.

It is further worth mentioning that the same principle can be appliedwhen symbols are characterized by a particular area in the CEI diagram,e.g. areas 120.1 or 120.2 in FIG. 4b . in particular, with reference toFIG. 4b , one can characterize a particular symbol by its duration andby having a color inside either one defined area or inside a combinationof different defined areas.

As an example, a symbol S0 could be characterized by its duration T0 anda color in the area 120.1 or 120.3, while a symbol S1 could becharacterized by its duration T1 and a color in the area 120.2 or 120.4.As will be understood by the skilled person, the transmission of symbolsS0 and S1 using this definition would facilitate maintaining a desiredcolor set point such as color set point 105 during the transmission. Asa consequence, the need to apply a color compensation scheme may becomesmaller.

In such embodiment, i.e. whereby a particular symbol can be transmittedby either one of a plurality of color, the following operating schemecan be applied:

When the current modulation schemes of the different LEDs or LED groupsare determined that need be applied to generated the desired VLC code,the average color that will be generated when the current modulationschemes are applied, can be monitored or determined.

Assuming that the required current schemes for the transmission of Msymbols have been determined and that such transmission would result inthe generation of particular color CX, CX e.g. corresponding to theaverage color generated during the transmission of the last N symbols ofthe M symbols, N e.g. being equal to M or smaller than M. Assuming thatthis color CX deviates from a desired color Cd, Cd e.g. beingrepresented by a color set point. At this point, one may determine thecurrent scheme for transmitting symbol M+1 by:

selecting the most appropriate color for the transmission of symbol M+1and,

determining the required modulation scheme.

In an embodiment of the present invention, the step of selecting themost appropriate color for the transmission of the symbol M+1 may bebased on the average color CX and the desired color Cd. In particular,the color for transmitting symbol M+1 may be selected so as to diminishthe difference between CX and Cd, or, phrased differently, select thecolor for transmitting the symbol M+1 ins such manner that a new valuefor CX, CX now being the average color of the last N symbols of the M+1symbols becomes closer to Cd.

Note that the average color as defined may be considered a movingaverage color, corresponding the average color generated during thetransmission of the last N symbols. Note that in a similar manner, amoving average color may also be determined as the average colorgenerated during a previous transmission period having a fixedpredetermined duration or period. In an embodiment, such a predeterminedduration or period may be associated with a frequency at which anobserver can notice color variations.

By applying such an operating scheme, one may e.g. be able to generatean average color or moving average color corresponding or substantiallycorresponding to the desired color.

It can be noted that the described operating scheme can be performed persymbol or per code word.

It is submitted that such operating scheme to determine the mostappropriate color for transmitting a particular symbol or code word maye.g. be performed in advance, i.e. prior to transmitting a particularVLC code. In such embodiment, the communication method according to thepresent invention may thus comprises determining current modulationschemes for each LED or LED groups of a plurality of LEDs or LED groupsin such manner that, during the transmission of the VLC code, i.e.during the application of the current modulation schemes, the averagecolor or moving average color as generated substantially corresponds toa desired color, e.g. represented by a color set point.

Alternatively, the operating scheme can be executed in real time. Insuch a real time implementation one can e.g. in real time, keep recordof the average color or moving average color as generated so far and,upon receipt of a new symbol or code word to be transmitted, select themost appropriate color and determine the associated current modulationscheme for transmitting the symbol or code word.

It can further be noted that, in case a sufficient correspondencebetween the moving average color CX as generated and the desired colorCd would not be feasible, a color compensation scheme as discussed maybe implemented.

As discussed above, in an embodiment of the present invention, a symbolor code word that is to be transmitted using a visual communicationmethod according to the present invention, may be characterized by botha duration or period and a color or color characteristic. In anembodiment, as discussed above, the color characterizing a symbol mayrefer to the average color used during the transmission of the symbol orthe code word.

In an alternative embodiment, a symbol may be characterized by the typeor kind of LED used to generate the modulated current profile. In suchembodiments, the color or color characteristic of the LED or LED groupthat is used to obtain a modulation on the combined current profile maydetermine which symbol is transmitted. This method can be bestunderstood with reference to FIG. 3.

In FIG. 3, modulation schemes for a current I1 and a current I2 areshown, whereby symbols can be determined based on the duration. In thescheme as shown, current I1 is kept constant, while current I2 ismodulated such that the combined current profile comprises a modulationwhich can be observed by a receiver and used to determine which symbolsare transmitted. It can further be noted that the average current I2over a symbol, e.g. S0, is the same as the average current I1 over thatsymbol. As such, with respect to the generated illumination, the sameillumination is assumed to be generated when current I1 would bemodulated and current I2 would be kept constant. In accordance with anembodiment of the present invention, these two modulations (eithermodulating current I1 and keeping current I2 constant or modulatingcurrent I2 and keeping current I1 constant) may however be considered todefine different symbols. In a similar manner, one could even opt tomodulate both I1 and I2 and this would then represent a third symbol. Assuch, in case current I1 and I2 are supplied to LEDs or LED groupshaving a different color or color characteristic, different symbols maybe defined, each having the same duration, depending on the LED or LEDgroups that is supplied with a modulated current.

Note that in general, unlike the example shown in FIG. 3, the average ofcurrent I1 will not be the same as the average of current I2, or, thedesired color would not correspond to the color as generated by an equalaverage current I1 and I2. In such situations, color compensatingschemes as discussed above may need to be implemented in order toarrive, on average, at a desired color, e.g. represented by a receivedcolor set point.

During the transmission via VLC, there are a large number ofrequirements that need to be met in high end illumination systems. Asmentioned above, in order to realize the VLC, a sufficient modulationdepth is required. Further, in order avoid flicker, which could causenausea for certain observers, the frequency content of the intensitymodulations should be sufficiently high. In addition, during the VLC, adesired illumination and color set-point should be preserved.

It would also be preferred that the mentioned requirements aremaintained also during transients. Within the meaning of the presentinvention, transients may e.g. include a transition between a firstillumination set-point and a second illumination set-point, anillumination set-point e.g. being characterized by a desiredillumination intensity and an illumination color. During such atransition, it would be desirable to maintain the requirements, e.g.w.r.t. modulation depth, for using VLC, such that a transition between afirst illumination set-point and a second illumination set-point can beperformed without interruption of the VLC, i.e. the visible lightcommunication. In addition, such a transient should be flicker-free,i.e. the transient should not be visible A transition for a non-VLC modeto a VLC mode or vice versa may also be considered a transient.

In an embodiment of the present invention, a transition from a firstillumination set-point to a second illumination set-point, during VLC isperformed by gradually adjusting the current profile, in particular byonly adjusting, when a next code word is transmitted, the currentprofile of only some of the symbols that make up the code word, i.e. notall the symbols, in a first step, upon receipt of a set-pointrepresenting a different illumination characteristic as represented bythe current profile, only the current profile of some of the symbols. Ina second step, the current profile of other symbols may be adjusted, toarrive more closely to the desired second illumination set-point.

By doing so, a gradual transition from a first illumination set-point toa second set-point is realized, thereby maintaining an accurate VLCoperation.

In an embodiment of the present invention, such a gradual adjustment isrealized by means of transitioning via one or more intermediate currentprofiles.

Such embodiment may e.g. be realized by means of a control unit whichmay be applied to control an LED driver configured to drive a pluralityof LEDs, whereby the control unit is configured to:

generate a current profile for each of the plurality of LEDs to generatethe desired illumination characteristic represented by a receivedset-point, whereby an envelope of the combined current profile ismodulated in order for a corresponding intensity variation as perceivedby a sensor to represent a variable length VLC code comprising codewords, a code word comprising a plurality of symbols, and wherein thecontrol unit is further configure to:

receive a further set-point representing a different desiredillumination characteristic as represented by the set-point, determine afurther current profile for one or more of the plurality of LEDs togenerate the different desired illumination characteristic representedby the further set-point, and

determine one or more intermediate current profiles for one or more ofthe plurality of LEDs, and

output control signals for the LED driver to sequentially drive theplurality of LEDs according to:

the current profiles when transmitting a first code word;

the one or more intermediate current profiles, when transmitting arespective one or more further code words, and

the further current profiles when transmitting a yet further code word.

By controlling the plurality of LEDs in such manner, a gradualtransition form a current profile to a further current profile isrealized via one or more intermediate current profiles, whereby thecurrent profile is adjusted per code word that is to be transmitted. Byapplying such a gradual transition between a first illuminationset-point and a second illumination set-point, the transmission of VLCcan more easily or more accurately sustained.

In an embodiment, care is taken that the modulation depth is notdecreased during the transition from a first intensity/color set-pointto a second intensity/color set-point.

In an embodiment, the current profile is adjusted proportional for eachof the LEDs of the plurality of LEDs. By doing so, the color set-pointmay be maintained during the transient.

In the current profiles in FIGS. 3 and 4, one can recognize that thecurrent may be adjusted at only predetermined instants spaced apart byincremental period W, with predetermined amounts, spaced apart by theincremental current step ICS. The smallest current increment that can beapplied to a current profile is thus to increase or decrease the currentwith the incremental current step ICS during a period equal to theincremental period W. This smallest increment is referred to as theminimal period current increment.

FIG. 5 schematically shows the current profiles for a first code word(consisting of symbols S0,S1,S2,S3), followed by a current profile fortransmitting the same code word, whereby a positive minimal periodcurrent increment M+ is applied to the second profile, in particular atthe start of the symbol S2.

In order to gradually decrease the intensity, in an embodiment of thepresent invention, the current profile of a next to be transmitted codeword is adjusted by applying a negative minimal period currentincrement. Such an embodiment is shown in FIG. 6. FIG. 6 schematicallyshows the current profiles for a first code word (consisting of symbolsS0,S1,S2,S3), followed by a current profile for transmitting the samecode word, whereby a negative minimal period current increment M− isapplied to the second profile, in particular at the end of the highcurrent portion of symbol S2.

In an embodiment of the present invention, the current profile of theplurality of LEDs is adjusted by only one minimal period currentincrement per code word that is transmitted.

In an embodiment of the present invention, a symbol is characterized byits length or duration in time, or period. In the VLC code as applied inthe present invention, a symbol may thus be characterized or identifiedby having a predetermined period or length in time. When a sequence ofsymbols is to be transmitted, the end of a symbol, corresponding to thestart of a next symbol, is triggered by a step (either positive ornegative) function. Such a step function may be realized in variousmanners. In the embodiments as shown in FIGS. 2,3,4, the step functionis realized by applying a substantial increase in the total currentprofile at the start of a next symbol, thus indicating the end of asymbol. In order to realize this, the symbols as shown in FIGS. 2,3,4each start with a comparatively high total current portion HCP, totalcurrent referring to the summed current of all the LED currents, and endwith a comparatively low total current portion LCP. Phrased differently,each of the plurality of symbols that is to be transmitted can becharacterized by a respective predetermined period, e.g. period S0, S1,S2, S3. In order for a sensor to identify these symbols, the combinedcurrent profile of the plurality of LEDs of LED groups applied ismodulated in such manner that during each of the predetermined periods,the combined current profile has a high current period, e.g. the hightotal current portion HCP, and a low current period, e.g. the low totalcurrent portion LCP.

In an embodiment, the total current or combined current of the currentprofiles, or the envelope of the combined current is such that the totalor combined current during the high current periods is substantially thesame for all symbols, or all words that are to be transmitted and issuch that the total or combined current during the low current periodsis substantially the same for all symbols, or all words that are to betransmitted.

Alternatively, the high current portions of the total or combinedcurrent may be different for different symbols in a code word.Similarly, the low current portions of the total or combined current maybe different for different symbols in a code word.

In the embodiments as shown in FIGS. 2, 3, 4, a transition from a highcurrent period to a low current period within the predetermined periodrepresenting a symbol or from a low current period to a high currentperiod within the predetermined period representing a symbol can beconsidered a step function. Alternatively, a more gradually transitionmay be applied as well. During such a more gradual transition, thetransition from a high current level to a low current level can e.g.take place over multiple incremental periods or clock periods, such asthe period W shown in FIG. 2.

In the embodiments shown in FIGS. 2,3,4, the end of a symbol can bedetected as the occurrence of a positive step, i.e. an increase of thetotal current. In particular, referring to FIG. 3, the total current asapplied to the plurality of LEDs increases from a comparatively lowvalue (value I1) to a comparatively high value (value I1+I2) at the endof the symbols S0, S1, S2, S3. The low total current value and the hightotal current value should be selected in such manner that theassociated intensity variation is sufficiently large that it can bedetected by a sensor that is to receive the VLC.

Alternatively, one could trigger the end of a symbol by a negative stepfunction, i.e. by applying a substantial decrease in the current symbolat the end of a symbol. FIG. 7 schematically illustrates differentsymbols S4, S5, S6, S7 that are represented in this manner.

In the embodiment as shown, the end of the symbols S4, S5, S6 and S7corresponds to the instants where the current I2 is reduced to zero,i.e. at instants t1, t2, t3 and t4.

At these instants, there will be an important decrease in the intensityof the illumination as provided by the LEDs, such decrease may bedetected by a sensor, thus enabling the determination of the duration orperiods of the different symbols.

With respect to adjusting of an intensity and/or color set-point duringoperation in a VLC mode, i.e. during visible light communication, it canbe pointed out that such an adjustment may also be realized by adjustingthe relative duration of the high current portion HCP and the lowcurrent portion LCP.

This is schematically illustrated in FIG. 8. FIG. 8 schematicallyillustrates the same symbols S4, S5, S6, S7 of FIG. 7, whereby, comparedto FIG. 7, the HCP of both symbols S4 and S7 is decreased.

Increasing or decreasing the relative width or duration of the highcurrent portion or period and the low current portion or period thusprovides in an additional way, compared to increasing or decreasing thecurrent amplitude, of adjusting an intensity of a light source, duringthe operation in VLC mode.

With respect to increasing or decreasing an amplitude of the current,e.g. either current I1 or I2 during a part of the HCP, as illustrated inFIGS. 5 and 6, it can be pointed out that it may be advantageous, forthe symbols that start with a HCP, to implement a positive currentincrement, such as the positive minimal period current increment M+shown in FIG. 5, at the beginning of the HCP of a symbol and toimplement a negative current increment, such as the negative minimalperiod current increment M− shown in FIG. 6, at the end of the HCP of asymbol.

In case of the application of the positive minimal period currentincrement M+ shown in FIG. 5 at the beginning of the symbol S1, thisincrement result in an increase in the modulation depth MD for thissymbol. As can be seen, the modulation depth MD for symbol S1 is largerthan then the modulation depth MD for symbol S0. Applying a negativeminimal period current increment M−, as shown in FIG. 6, at the end ofthe HCP of a symbol enables to avoid that the modulation depth wouldbecome smaller.

As indicated above, in order to adjust an intensity of the light emittedby the LEDs or LED groups, the current profiles can be adjusted, eitherby adjusting an amplitude of the current, e.g. an amplitude of a currentprofile, or by adjusting a width of a portion of the current profile. Inan embodiment, the current profiles are adjusted such that the frequencycontent of the profile is not lowered, i.e. care is taken that no lowerfrequencies are introduced by determining the adjusted or intermediatecurrent profiles. When considering this, it may e.g. be preferred tointroduce one or more current increments to a current profile, ratherthan widening or narrowing an HCP of a current profile.

The application of current increments or the application of gradualtransitions between current levels or combined or total current levelsenables to adjust or control the frequency content of the currentprofile or total current profile as applied. Controlling the frequencycontent of the applied current profile enables to control the occurrenceof flicker as observed by a human observer. According to a second aspectof the present invention, there is thus provided a method oftransmitting data using a variable length VLC code, wherein thesecurrent increments or gradual transitions are implemented.

In an embodiment, the present invention provides in an improved methodof transmitting data using a variable length VLC code in that in enablesto mitigate the occurrence of flicker, in particular during a VLCtransmission, by the application of current increments to the currentprofiles representing the symbols that are transmitted, or theapplication of gradual transitions between current levels or combined ortotal current levels.

As illustrated in FIG. 2, data or information may be coded into an arrayof symbols, whereby a particular combination of symbols may be construedas words. In the variable length VLC code according to the presentinvention, symbols are identifiable, as illustrated in FIG. 2, by theirlength, whereby the beginning and ending of symbols may be noticed by a(steep) increase or decrease of the generated intensity of the pluralityof LEDs applied, the combined current profile being representative ofthe generated intensity.

In the method according to the present invention, particular measuresmay be taken to ensure that the transmission of the VLC code issatisfactory, under various circumstances, e.g. various illuminationset-points, whereby illumination may refer to either color or intensityor both. As indicated above, in order to identify the VLC code that istransmitted, intensity variations of the light as perceived by a sensor,e.g. a sensor in a user equipment such as a smartphone, are relied upon.Such variations, which can e.g. be quantified by the aforementionedmodulation depth, should be sufficiently high in order for a processingunit, e.g. a processing unit of a user equipment that needs to receivethe VLC signal, to determine the beginning and end of the varioussymbols that are transmitted. Typically, such a user equipment, e.g. asmartphone or the like, would comprise a sensor that is configured todetect the illumination that is transmitted by the plurality of LEDs anda processing unit configured to process an illumination signal receivedform the sensor, the illumination signal representing the detectedillumination. Based on the illumination signal, the processing unit maydetermine the beginning and end of the various symbols that aretransmitted. This can e.g. be done using threshold detection but canalso be done using fast fourier transform. Such processing of a receivedsensor signal can e.g. comprise the filtering out of a DC level of theillumination signal and perform a processing of the remaining filtered(AC) signal.

In an embodiment, the envelope of the current profile of the differentLED groups is such that it comprises a modulation that can be detectedby a user equipment comprising a sensor and a processing unit and issufficiently large so as to determine which symbol it represents. In anembodiment, rather than each symbol having an HCP and an LCP, each witha substantially constant current, a current increment is applied, eitherat the HCP or at the LCP or both. Such a current increment can beapplied on each symbol of a word or on only a subset of symbols of aword, e.g. on only one symbol.

In an embodiment, the current increment is applied to the longestsymbol.

FIG. 9 schematically depicts various possible modulations as can beapplied in a method according to the present invention. The possiblemodulations as shown can be considered modifications to the modulationas shown in FIG. 2.

In the current profile shown in FIG. 9(a), a positive current incrementPCI is applied in the high current portion HCP of the current profileCP.

In the current profile shown in FIG. 9(b), a positive PCI and a negativecurrent increment NCI are applied.

In the current profile shown in FIG. 9(c), a negative current incrementNCI is applied and a gradual transition between the HCP and the LCP viatwo intermediate current levels Im1 and Im2.

In an embodiment, the current modulation is realized with only one LEDgroup of the plurality of LED groups. Phrased differently, in suchembodiment, the modulation depth may entirely be attributed to atransition of a current supplied to one of the LEDs or LED groups from acomparatively low current (e.g. zero) to a comparatively high current,while the currents supplied to the other LED or LED groups are keptsubstantially constant.

Such a situation is e.g. depicted in FIG. 3, whereby the modulationdepth is entirely determined by the modulation of the current I2.

In an embodiment of the present invention, it is assumed that thecurrents as supplied to the plurality of LEDs or LED groups may beindependently controlled. Such an arrangement may e.g. allow to supply afirst LED group with a modulated current, e.g. switching between a highlevel and a low level, and to supply a second LED group with asubstantially constant current.

Such an embodiment may e.g. be implemented in case the desired intensityis comparatively high. In such case, the required modulation depth for asuccessful VLC operation may be realized with only one LED or LED groupof the plurality of LEDs or LED groups.

As such, in an embodiment of the present invention, the modulationapplied to the current envelope representing the combined currentprofiles of the LEDs or LED groups is realized with only one LED or LEDgroup of the plurality of LEDs or LED groups, in case the desiredintensity is above a predefined threshold.

In such a situation, additional criteria such as efficiency or anaccurate representation of a desired color or avoiding the occurrence offlicker may be considered as well, in addition to the requirement ofenabling the operation in VLC mode.

As such, in an embodiment, of the present invention, current incrementsas e.g. shown in FIGS. 9a-c are applied to the current profiles toobtain a desired color set-point, in case the desired intensity, e.g.derived from a received illumination set-point, is above a predefinedvalue or threshold.

In an embodiment of the present invention, a desired color set-point orintensity set-point need not be realized for each symbol of a code wordseparately; rather, it may be sufficient that the current profiles asapplied for transmitting a code word, i.e. a combination of a pluralityof symbols, are such that a desired color set-point or intensityset-point is realized. By doing so, a desired color set-point orintensity set-point can be more accurately obtained. I.e. a higherresolution w.r.t. color and/or intensity can be realized. In suchembodiment, the use of one or more current increments to one or moresymbols of a code word that is to be transmitted further increases thepossibilities to generate a desired color set-point or intensityset-point.

In an embodiment, different current increments are applied to differentsymbols of a word. As an example, a positive current increment may beapplied on the first symbol, whereas a negative current increment can beapplied on a second symbol of a word.

The current increment may be applied on only one current or currentprofile or on multiple currents or current profiles.

In order to ensure a proper detection or demodulation of the VLC signal,the modulation depth should be sufficiently large. In general, asufficiently large modulation depth may be realized comparatively easywhen the intensity set-point is rather high.

At comparatively low intensity set-points, the modulation depth may e.g.be realized by providing multiple currents or current profiles with anHCP followed by an LCP. In an embodiment, the transitions from an HCP toan LCP for the multiple currents may be synchronized.

FIG. 10 schematically shows a current profile of a first current I1,e.g. supplied to a first LED or LED group and a current profile of asecond current I2, e.g. supplied to a second LED or LED group and theenvelope of the resulting current Itot. In an embodiment, the first LEDor LED group may have different characteristics that the second LED orLED groups, e.g. a different color spectrum. Each of the currents I1 andI2 has a profile with a high current portion HCP followed by a lowcurrent portion LCP, in a period T, e.g. representing a particularsymbol in a variable length VLC code. As a result, each current may beconsidered to have a modulation depth, MD1, resp. MD2. The resultingcurrent Itot will have the combined modulation depth MD, which may e.g.be sufficiently large for a user equipment, in particular a sensor of auser equipment, to detect the intensity variation associated with thecurrent variation, so as to determine the length, duration or period T.

In this respect, it can be noted that, in general, the conversion fromcurrent to light and the sensitivity of the human eye (or of anillumination sensor) will not be equal for all types of LED and allcolors (spectra). These differences may e.g. be taken into account whendetermining the current profiles applied for the different LED groups.In order to implement this, an LED driver according to the presentinvention may be provided with a sensitivity characteristic or functionof one or more illumination sensors, the characteristic indicating thesensor's sensitivity as a function of frequency. Using such information,it may e.g. be advantageously to implement the largest modulation stepin a current for an LED or LED group having a color spectrum for whichthe sensor is the most sensitive.

By applying current transitions (e.g. MD1, MD2) in multiple currentprofiles, whereby the summed transition serves as an overall modulationdepth (MD), operating in VLC mode may even be possible at comparativelylow intensities. Note that, in order to ensure proper operation in VLCmode, i.e. ensuring a sufficiently large modulation depth MD, one mayneed to make concessions with respect to other criteria that may need tobe met, e.g. a desired color set-point.

Generalizing this, the skilled person may understand that, apart fromthe realization of a particular illumination set-point, e.g. specifiedby a desired intensity and color, other criteria or requirements may bedesired as well. Such criteria may e.g. include the avoidance offlicker, driver efficiency, etc. In case a VLC code is to be transmittedby modulating the intensity of the generated light, the realization ofthe various requirements may come into conflict with each other. Phraseddifferently, not all required or desired characteristics may berealized; a selection or prioritizing of the goals to be achieved may berequired.

As an example, it may be difficult to realize a desired modulation depthwhen the desired intensity of the generated light drops below a certainvalue. In such an embodiment, a desired color set-point is onlyapproximated when the desired intensity is below a certain,predetermined or predefined threshold and operating in VLC mode isrequired.

On the other hand, in case the desired intensity is comparatively high,it may be comparatively easy to realize a desired modulation depth, i.e.the desired or required illumination depth may be realized in differentmanners. In such circumstances one may e.g. use this operational freedomto further optimize the operation of e.g. the LED driver supplying thepower to the LEDs or LED groups. In an embodiment, such an optimizationmay include optimizing the operation of the LED driver towardsefficiency. Alternatively, the operation of the LED driver may beoptimized towards the most accurate realization of a desired colorset-point.

In accordance with an aspect of the present invention, the modulationscheme of the current or currents supplied to a plurality of LEDs or LEDgroups is such that priority is given in attaining the visible lightcommunication, while mitigating the occurrence of flicker. In suchembodiment, the desired illumination set-point, e.g. characterized by anintensity and a color, may not be accurately generated, but onlyapproximated. In such situation, priority may e.g. be given togenerating light with the desired intensity combined with a currentmodulation with a sufficient modulation depth. In order to generate thissufficient modulation depth, concessions with respect to color may haveto be made.

In an embodiment, the present invention provides in an LED driver thatis configured to perform the above described methods of operating aplurality of LEDs or LED groups, in order to transmit a VLC code.

In such embodiment, the LED driver may comprise a control unit toreceive an illumination set-point and data to be transmitted. Such acontrol unit may further be configured to determine the required currentprofiles and generate control signals for a power converter of the LEDdriver, in order to generate the required current profiles.

In an embodiment, such an LED driver may be configured to operate inthree different operating modes, depending on an intensity set-point.

In a first operating mode, characterized by the intensity set-pointbeing below a first predetermined value or threshold, priority is givento obtaining a required modulation depth for transmitting a VLC code,irrespective of a desired color set-point or other criteria.

In a second operating mode, characterized by the intensity set-pointbeing above a second predetermined value or threshold, the LED drivermay be configured to consider a further criterion to be met, apart fromthe illumination set-point. Such a criterion may e.g. be an optimizationof the efficiency of the LED driver, or an optimization w.r.t. flicker;i.e. optimizing the current profiles of the LEDs or LED groups such thatflicker is minimized.

When operating in the second operating mode, the LED driver, inparticular the control unit of the LED driver, may e.g. be configured toapply a plurality of current increments to the plurality of symbols thatform a code word, thereby increasing a frequency content of the currentprofile or current profiles. Such measures have been found to mitigateor avoid the occurrence of flicker. The application of one or morecurrent increments may further enable to accurately obtain the desiredcolor set-point. At the same time, the current profiles may be selectedso as to optimize efficiency of the LED driver. An optimization of theefficiency may e.g. be realized by considering one or more of thefollowing:

an overall efficiency characteristic of the LED driver driving theplurality of LEDs or LED groups;

switching losses in the switches controlling the current through theLEDs or LED groups;

illumination characteristics of the LEDs or LED groups (i.e. current vs.illumination for the LEDs or LED groups.

In a third operating mode, characterized by the intensity being inbetween the first threshold and the second threshold, the LED driver maye.g. be configured to focus of obtained the desired illuminationset-point and to avoid or mitigate flicker during VLC transmission,rather than optimizing the efficiency.

The LED driver according to the present invention may be configured tobe combined with an LED fixture comprising LEDs or LED groups forrealizing desired illumination set-points.

Examples of such LED fixtures may e.g. comprise LEDs or LED groupsemitting different colors or different color spectra. Such fixtures maye.g. include RGBW (red, green, blue, white) or RGBA (red, green blue,amber), RGBWA (red, green, blue, white, amber), RGB UV (red, green,blue, ultra-violet) LED groups, or combinations thereof.

LED fixtures may also comprise different types of white LEDs, e.g. acombination of WW (warm white) LEDs and CW (cold white) LEDs.

In an embodiment, an LED driver according to the present inventioncomprises one or more power converters, e.g. switched mode powerconverters such as Buck or Boost converters.

Such an LED driver is schematically shown in FIG. 11. FIG. 11schematically shows an LED driver 710 according to the presentinvention. The LED driver may e.g. be connected to a light engine 720comprising two LED groups 722 and 724. In the embodiment as shown, theLED driver 710 may be configured to output three currents Ia, Ib, Ic, atrespective output terminals 710.1, 710.2 and 710.3, the output currentsbeing provided by three power converters, e.g. switched mode powerconverters, 750.1, 750.2 and 750.3 of the LED driver. The LED driver 710further comprises a control unit 730 which may be configured to provide,at an output terminal 730.2, control signals 730.3 to control the powerconverters of the LED driver, the control signal e.g. representing thedesired current profiles. The control unit 730 further comprises aninput terminal 730.1 configured to receive an input signalrepresentative of a desired illumination set-point and data that is tobe transmitted via VLC. In the embodiment as shown, the output terminals710.1, 710.2 of the LED driver 710 may be connected to the inputterminals 720.1, 720.2 of the light engine 720 to power the LED groups724 and 722. As such, in the embodiment as shown, currents Ia and Ib ofthe LED driver may be provided, to LED groups 722 and 724 of the lightengine.

As required, detailed embodiments of the present invention are disclosedherein;

By means of the present invention, a desired illumination set-point(e.g. defined by a color and intensity set-point) can be realised, evenduring the application of a VLC communication. The particularapplication of current increments on one or more of the symbols that areto be transmitted enable an accurate realisation of a desiredillumination set-point, despite limitations w.r.t. available currentvalues or available instants to adjust the current. By realising adesired illumination set-point when considering an entire code word,rather than considering each symbol separately, an improved resolution,with respect to intensity and color is realised. The application of theone or more current increments further enables to avoid or mitigate theadverse effects of flicker to the human observer.

However, it is to be understood that the disclosed embodiments aremerely exemplary of the invention, which can be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure. Further, the terms and phrases usedherein are not intended to be limiting, but rather, to provide anunderstandable description of the invention.

The terms “a” or “an”, as used herein, are defined as one or more thanone. The term plurality, as used herein, is defined as two or more thantwo. The term another, as used herein, is defined as at least a secondor more. The terms including and/or having, as used herein, are definedas comprising (i.e., open language, not excluding other elements orsteps). Any reference signs in the claims should not be construed aslimiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

The term coupled, as used herein, is defined as connected, although notnecessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of severalitems recited in the claims.

The terms program, software application, and the like as used herein,are defined as a sequence of instructions designed for execution on acomputer system. A program, computer program, or software applicationmay include a subroutine, a function, a procedure, an object method, anobject implementation, an executable application, an applet, a servlet,a source code, an object code, a shared library/dynamic load libraryand/or other sequence of instructions designed for execution on acomputer system.

A computer program may be stored and/or distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems.

The invention claimed is:
 1. A visual light communication (VLC) methodfor a lighting system comprising an LED driver and a plurality of LEDsor LED groups, the method comprising the steps of: receiving a set-pointrepresenting a desired illumination characteristic and data that is tobe transmitted using VLC; determining a current profile for each LED orLED group of the plurality of LEDs or LED groups based on the desiredillumination characteristic represented by the set-point, whereby anenvelope of a combined current profile of the current profiles ismodulated in order for a corresponding intensity variation as perceivedby a sensor to represent a variable length VLC code representing thedata, the VLC code comprising one or more code words, a code wordcomprising a plurality of symbols, whereby each of the plurality ofsymbols is characterized by a respective predetermined period; andgenerating one or more control signals to drive the plurality of LEDs orLED groups in accordance with the current profiles in order for theplurality of LEDs or LED groups to transmit the VLC, wherein thecombined current profile has a high current period and a low currentperiod during each predetermined period and wherein the combined currentprofile comprises, during the high current period or the low currentperiod of at least one of the predetermined periods characterizing asymbol, at least one current increment during a current increment periodthat is less than the high current period or the low current period. 2.The visual light communication method according to claim 1, wherein thestep of determining a current profile for each of the plurality of LEDsor LED groups comprises determining a modulated current profile for atleast one LED or LED group of the plurality of LEDs or LED groups. 3.The visual light communication method according to claim 2, wherein amodulation of the combined current profile corresponds to a modulationof the modulated current profile for the at least one LED or LED groupof the plurality of LEDs or LED groups.
 4. The visual lightcommunication method according to claim 1, wherein the combined currentprofile comprises a summation of the current profiles of the pluralityof LEDs or LED groups.
 5. The visual light communication methodaccording to claim 2, wherein the plurality of LEDs or LED groupscomprise LEDs or LED groups having a different color and the at leastone LED or LED group of the plurality of LEDs or LED groups is selectedbased on the data.
 6. The visual light communication method according toclaim 1, wherein the data is encoded in the envelope of the combinedcurrent profile.
 7. The visual light communication method according toclaim 1, wherein the at least one current implement is applied to one ormore current profiles.
 8. The visual light communication methodaccording to claim 1, wherein a difference between an average currentduring the high current portion and an average current during the lowcurrent portion is above a predetermined value.
 9. The visual lightcommunication method according to claim 1, wherein a current incrementis applied on a subset of the symbols of a code word.
 10. The visuallight communication method according to claim 1, wherein a positivecurrent increment is applied on a subset of the symbols and a negativecurrent increment is applied on another subset of the symbols of thecode word.
 11. The visual light communication method according to claim1, wherein the current increment starts at the beginning of a symbol.12. The visual light communication method according to claim 1, whereinthe current implement is applied symmetrically about a central positionof the high current period or the low current period.
 13. The visuallight communication method according to claim 1, wherein the highcurrent period proceeds the low current period.
 14. The visual lightcommunication method according to claim 1, wherein a transition betweenthe high current period and the low current period takes longer than apredetermined period.
 15. The visual light communication methodaccording to claim 14, wherein the predetermined period is anincremental period associated with a clock frequency.
 16. The visuallight communication method according to claim 1, wherein the currentincrement corresponds to a minimal period current increment.
 17. An LEDdriver for driving a plurality of LEDs or LED groups to transmit lightin a VLC mode, wherein the LED driver comprises a control unit that isconfigured to perform the method according to claim
 1. 18. A visuallight communication (VLC) method for a lighting system comprising an LEDdriver and a plurality of LEDs or LED groups, the method comprising thesteps of: receiving a set-point representing a desired illuminationcharacteristic and data that is to be transmitted using VLC; determininga current profile for each LED or LED group of the plurality of LEDs orLED groups based on the desired illumination characteristic representedby the set-point, whereby an envelope of a combined current profile ofthe current profiles is modulated in order for a corresponding intensityvariation as perceived by a sensor to represent a variable length VLCcode representing the data, the VLC code comprising one or more codewords, a code word comprising a plurality of symbols, whereby each ofthe plurality of symbols is characterized by a respective predeterminedperiod; generating one or more control signals to drive the plurality ofLEDs or LED groups in accordance with the current profiles in order forthe plurality of LEDs or LED groups to transmit the VLC; receiving afurther set-point representing a different desired illuminationcharacteristic; determining a further current profile for one or more ofthe plurality of LEDs to generate the different desired illuminationcharacteristic represented by the further set-point; and determining oneor more intermediate current profiles for one or more of the pluralityof LEDs, and determining output control signals for the LED driver tosequentially drive the plurality of LEDs or LED groups according to: thecurrent profiles when transmitting a first code word; the one or moreintermediate current profiles, when transmitting a respective one ormore further code words, and the further current profiles whentransmitting a yet further code word.
 19. An LED driver for driving aplurality of LEDs or LED groups to transmit light in a VLC mode, whereinthe LED driver comprises a control unit that is configured to performthe method according to claim
 18. 20. An LED driver for driving aplurality of LEDs or LED groups to transmit light in a VLC mode, whereinthe LED driver comprises a control unit that is configured to: receive,at an input terminal, a set-point representing a desired illuminationcharacteristic and data that is to be transmitted using VLC; determine acurrent profile for each LED or LED group of the plurality of LEDs orLED groups based on the desired illumination characteristic representedby the set-point, whereby an envelope of a combined current profile ofthe current profiles is modulated in order for a corresponding intensityvariation as perceived by a sensor to represent a variable length VLCcode representing the data, the VLC code comprising one or more codewords, a code word comprising a plurality of symbols, whereby each ofthe plurality of symbols is characterized by a respective predeterminedperiod; and generate one or more control signals to drive the pluralityof LEDs or LED groups in accordance with the current profiles in orderfor the plurality of LEDs or LED groups to transmit the VLC code,wherein the combined current profile has a high current period and a lowcurrent period during each predetermined period, and wherein thecombined current profile comprises, during the high current period orthe low current period of at least one of the predetermined periodscharacterizing a symbol, a current increment during a current incrementperiod that is less than the high current period or the low currentperiod.
 21. The LED driver according to claim 20, wherein the controlunit is configured to determine a modulated current profile for at leastone LED or LED group of the plurality of LEDs or LED groups.
 22. The LEDdriver according to claim 21, wherein a modulation of the combinedcurrent profile corresponds to a modulation of the modulated currentprofile for the at least one LED or LED group of the plurality of LEDsor LED groups.
 23. The LED driver according to claim 21, wherein theplurality of LEDs or LED groups comprise LEDs or LED groups having adifferent color and the at least one of the plurality of LEDs or LEDgroups is selected based on the data.
 24. The LED driver according toclaim 23, wherein the data is encoded in the envelope of the combinedcurrent profile.
 25. The LED driver according to claim 20, wherein thecombined current profile comprises a summation of the current profilesof the plurality of LEDs or LED groups.
 26. The LED driver according toclaim 20, wherein the at least one current implement is applied to oneor more current profiles.
 27. The LED driver according to claim 20,wherein a difference between an average current during the high currentportion and an average current during the low current portion is above apredetermined value.
 28. The LED driver according to claim 20, wherein acurrent increment is applied on a subset of the symbols of a code word.29. The LED driver according to claim 20, wherein a positive currentincrement is applied on a subset of the symbols and a negative currentincrement is applied on another subset of the symbols of the code word.30. The LED driver according to claim 20, wherein the current incrementstarts at the beginning of a symbol.
 31. The LED driver according toclaim 20, wherein the current implement is applied symmetrically about acentral position of the high current period or the low current period.32. The LED driver according to claim 20, wherein the high currentperiod proceeds the low current period.
 33. The LED driver according toclaim 20, wherein a transition between the high current period and thelow current period takes longer than a predetermined period.
 34. The LEDdriver according to claim 33, wherein the predetermined period is anincremental period associated with a clock frequency.
 35. The LED driveraccording to claim 20, wherein the current increment corresponds to aminimal period current increment.
 36. The LED driver according to claim20, wherein the LED driver comprises one or more switched mode powerconverters, configured to: receive the one or more control signals fromthe control unit and generate output currents for the plurality of LEDsor LED groups in accordance with the determined current profiles oradjusted current profile or profiles.
 37. An LED driver for driving aplurality of LEDs or LED groups to transmit light in a VLC mode, whereinthe LED driver comprises a control unit that is configured to: receive,at an input terminal, a set-point representing a desired illuminationcharacteristic and data that is to be transmitted using VLC; determine acurrent profile for each LED or LED group of the plurality of LEDs orLED groups based on the desired illumination characteristic representedby the set-point, whereby an envelope of a combined current profile ofthe current profiles is modulated in order for a corresponding intensityvariation as perceived by a sensor to represent a variable length VLCcode representing the data, the VLC code comprising one or more codewords, a code word comprising a plurality of symbols, whereby each ofthe plurality of symbols is characterized by a respective predeterminedperiod; generate one or more control signals to drive the plurality ofLEDs or LED groups in accordance with the current profiles in order forthe plurality of LEDs or LED groups to transmit the VLC code; uponreceipt of a further set-point representing a different illuminationcharacteristic during the transmission of the VLC code, determine anadjusted current profile or profiles for one or more of the plurality ofLEDs or LED groups based on the different illumination characteristic,whereby the current profile is only adjusted for some, not all, symbolsof a next code word to be transmitted; generate one or more controlsignals to drive the plurality of LEDs or LED groups in accordance withthe adjusted current profile or profiles, and output, via an outputterminal of the control unit, one or more control signals to drive theplurality of LEDs or LED groups in accordance with the adjusted currentprofile or profiles.
 38. The LED driver according to claim 37, whereinthe LED driver is configured to adjust a current for any LED of theplurality of LEDs or LED groups at a plurality of discrete values at aplurality of discrete instants.
 39. The LED driver according to claim38, wherein the LED driver is configured to adjust the current profileof only one symbol of a next code word with a minimum current incrementper instant.
 40. The LED driver according to claim 39, wherein eachsymbol is characterized by a predetermined period and at least twodifferent intensities applied during the predetermined period.
 41. TheLED driver according to claim 40, wherein each symbol is characterizedby application of a comparatively high intensity during a first portionof the predetermined period, followed by application of a comparativelylow intensity during a second portion of the predetermined period. 42.The LED driver according to claim 37, wherein the LED driver comprisesone or more switched mode power converters, configured to: receive theone or more control signals from the control unit and generate outputcurrents for the plurality of LEDs or LED groups in accordance with thedetermined current profiles or adjusted current profile or profiles. 43.A control unit for controlling an LED driver to drive a plurality ofLEDs or LED groups to transmit light in a VLC mode, the control unitcomprising: an input terminal configured to receive a set-pointrepresenting a desired illumination characteristic and data that is tobe transmitted using VLC; an output terminal configured to output one ormore control signals for controlling the LED driver; the control unitbeing configured to: determine a current profile for each LED or LEDgroup of the plurality of LEDs or LED groups to generate the desiredillumination characteristic represented by the set-point, whereby anenvelope of the combined current profile is modulated in order for acorresponding intensity variation as perceived by a sensor to representa variable length VLC code comprising code words, a code word comprisinga plurality of symbols; generate one or more control signals to drivethe plurality of LEDs or LED groups in accordance with the currentprofiles in order for the plurality of LEDs or LED groups to transmitthe VLC code; receive a further set-point representing a differentdesired illumination characteristic, determine a further current profilefor one or more LEDs or LED groups of the plurality of LEDs to generatethe different desired illumination characteristic represented by thefurther set-point; determine one or more intermediate current profilesfor one or more LEDs or LED groups of the plurality of LEDs or LEDgroups; and determine output control signals for the LED driver tosequentially drive the plurality of LEDs or LED groups according to: thecurrent profiles when transmitting a first code word; the one or moreintermediate current profiles, when transmitting a respective one ormore further code words, and the further current profiles whentransmitting a yet further code word.
 44. The control unit according toclaim 43, wherein each symbol is characterized by a predetermined periodand at least two different intensities applied during the predeterminedperiod.
 45. The control unit according to claim 44, wherein each symbolis characterized by application of a comparatively high intensity duringa first portion of the predetermined period, followed by application ofa comparatively low intensity during a second portion of thepredetermined period.